Diabetes mellitus is a serious illness that affects an increasing number of people across the world. Its incidence is escalating along with the increasing trend to obesity in many countries. The serious consequences of the disease include increased risk of stroke, heart disease, kidney damage, blindness, and amputation. Diabetes is characterized by decreased insulin secretion and/or an impaired ability of peripheral tissues to respond to insulin, resulting in increased plasma glucose levels. There are two forms of diabetes: insulin-dependent and non-insulin-dependent, with the great majority of diabetics suffering from the non-insulin-dependent form of the disease, known as type 2 diabetes or non-insulin-dependent diabetes mellitus (NIDDM). Because of the serious consequences, there is an urgent need to control diabetes.
Treatment of NIDDM generally starts with weight loss, a healthy diet and an exercise program. These factors are especially important in addressing the increased cardiovascular risks associated with diabetes, but they are generally ineffective in controlling the disease itself. There are a number of drug treatments available, including insulin, metformin, sulfonylureas, acarbose, and thiazolidinediones. However, each of these treatments has disadvantages, and there is an ongoing need for new drugs to treat diabetes.
Metformin is an effective agent that reduces fasting plasma glucose levels and enhances the insulin sensitivity of peripheral tissue. Metformin has a number of effects in vivo, including an increase in the synthesis of glycogen, the polymeric form in which glucose is stored [R. A. De Fronzo Drugs 1999, 58 Suppl. 1, 29]. Metformin also has beneficial effects on lipid profile, with favorable results on cardiovascular health—treatment with metformin leads to reductions in the levels of LDL cholesterol and triglycerides [S. E. Inzucchi JAMA 2002, 287, 360]. However, over a period of years, metformin loses its effectiveness [R. C. Turner et al. JAMA 1999, 281, 2005] and there is consequently a need for new treatments for diabetes.
Thiazolidinediones are activators of the nuclear receptor peroxisome-proliferator activated receptor-gamma. They are effective in reducing blood glucose levels, and their efficacy has been attributed primarily to decreasing insulin resistance in skeletal muscle [M. Tadayyon and S. A. Smith Expert Opin. Investig. Drugs 2003, 12, 307]. One disadvantage associated with the use of thiazolidinediones is weight gain.
Sulfonylureas bind to the sulfonylurea receptor on pancreatic beta cells, stimulate insulin secretion, and consequently reduce blood glucose levels. Weight gain is also associated with the use of sulfonylureas [S. E. Inzucchi JAMA 2002, 287, 360] and, like metformin, efficacy decreases over time [R. C. Turner et al. JAMA 1999, 281, 2005]. A further problem often encountered in patients treated with sulfonylureas is hypoglycemia [M. Salas J. J. and Caro Adv. Drug React. Tox. Rev. 2002, 21, 205-217].
Acarbose is an inhibitor of the enzyme alpha-glucosidase, which breaks down disaccharides and complex carbohydrates in the intestine. It has lower efficacy than metformin or the sulfonylureas, and it causes intestinal discomfort and diarrhea which often lead to the discontinuation of its use [S. E. Inzucchi JAMA 2002, 287, 360].
The metabolic syndrome is a condition where patients exhibit more than two of the following symptoms: obesity, hypertriglyceridemia, low levels of HDL-cholesterol, high blood pressure, and elevated fasting glucose levels. This syndrome is often a precursor of type 2 diabetes, and has high prevalence in the United States with an estimated prevalence of 24% (E. S. Ford et al. JAMA 2002, 287, 356). A therapeutic agent that ameliorates the metabolic syndrome would be useful in potentially slowing or stopping the progression to type 2 diabetes.
In the liver, glucose is produced by two different processes: gluconeogenesis, where new glucose is generated in a series of enzymatic reactions from pyruvate, and glycolysis, where glucose is generated by the breakdown of the polymer glycogen.
Two of the key enzymes in the process of gluconeogenesis are phosphoenolpyruvate carboxykinase (PEPCK) which catalyzes the conversion of oxalacetate to phosphoenolpyruvate, and glucose-6-phosphatase (G6 Pase) which catalyzes the hydrolysis of glucose-6-phosphate to give free glucose. The conversion of oxalacetate to phosphoenolpyruvate, catalyzed by PEPCK, is the rate-limiting step in gluconeogenesis. On fasting, both PEPCK and G6 Pase are upregulated, allowing the rate of gluconeogenesis to increase. The levels of these enzymes are controlled in part by the corticosteroid hormones (cortisol in human and corticosterone in mouse). When the corticosteroid binds to the corticosteroid receptor, a signaling cascade is triggered which results in the upregulation of these enzymes.
The corticosteroid hormones are found in the body along with their oxidized 11-dehydro counterparts (cortisone and II-dehydrocorticosterone in human and mouse, respectively), which do not have activity at the glucocorticoid receptor. The actions of the hormone depend on the local concentration in the tissue where the corticosteroid receptors are expressed. This local concentration can differ from the circulating levels of the hormone in plasma, because of the actions of redox enzymes in the tissues. The enzymes that modify the oxidation state of the hormones are 11beta-hydroxysteroid dehydrogenases forms I and II. Form I (11β-HSD1) is responsible for the reduction of cortisone to cortisol in vivo, while form II (11β-HSD2) is responsible for the oxidation of cortisol to cortisone. The enzymes have low homology and are expressed in different tissues. 11β-HSD1 is highly expressed in a number of tissues including liver, adipose tissue, and brain, while 11β-HSD2 is highly expressed in mineralocorticoid target tissues, such as kidney and colon. 11β-HSD2 prevents the binding of cortisol to the mineralocorticoid receptor, and defects in this enzyme have been found to be associated with the syndrome of apparent mineralocorticoid excess (AME).
Since the binding of the 11β-hydroxysteroids to the corticosteroid receptor leads to upregulation of PEPCK and therefore to increased blood glucose levels, inhibition of 11β-HSD1 is a promising approach for the treatment of diabetes. In addition to the bio-chemical discussion above, there is evidence from transgenic mice, and also from small clinical studies in humans, that confirm the therapeutic potential of the inhibition of 11β-HSD1.
Experiments with transgenic mice indicate that modulation of the activity of 11β-HSD1 could have beneficial therapeutic effects in diabetes and in the metabolic syndrome. For example, when the 11β-HSD1 gene is knocked out in mice, fasting does not lead to the normal increase in levels of G6 Pase and PEPCK, and the animals are not susceptible to stress- or obesity-related hyperglycemia. Moreover, knockout animals which are rendered obese on a high-fat diet have significantly lower fasting glucose levels than weight-matched controls (Y. Kotolevtsev et al. Proc. Natl. Acad. Sci. USA 1997, 94, 14924). 11β-HSD1 knockout mice have also been found to have improved lipid profile, insulin sensitivity, and glucose tolerance (N. M. Morton et al. J. Biol. Chem. 2001, 276, 41293). The effect of overexpressing the 11β-HSD1 gene in mice has also been studied. These transgenic mice displayed increased 11β-HSD1 activity in adipose tissue and exhibited visceral obesity which is associated with the metabolic syndrome. Levels of the corticosterone were increased in adipose tissue, but not in serum, and the mice had increased levels of obesity, especially when on a high-fat diet. Mice fed on low-fat diets were hyperglycemic and hyperinsulinemic, and also showed glucose intolerance and insulin resistance (H. Masuzaki et al. Science, 2001, 294, 2166).
The effects of the non-selective 11β-hydroxysteroid dehydrogenase inhibitor carbenoxolone have been studied in a number of small trials in humans. In one study, carbenoxolone was found to lead to an increase in whole body insulin sensitivity, and this increase was attributed to a decrease in hepatic glucose production (B. R. Walker et al. J. Clin. Endocrinol. Metab. 1995, 80, 3155). In another study, decreased glucose production and glycogenolysis in response to glucagon challenge were observed in diabetic but not healthy subjects (R. C. Andrews et al. J. Clin. Endocrinol. Metab. 2003, 88, 285). Finally, carbenoxolone was found to improve cognitive function in healthy elderly men and also in type 2 diabetics (T. C. Sandeep et al. Proc. Natl. Acad. Sci. USA 2004, 101, 6734).
A number of non-specific inhibitors of 11β-HSD1 and 11β-HSD2 have been identified, including glycyrrhetinic acid, abietic acid, and carbenoxolone. In addition, a number of selective inhibitors of 11β-HSD1 have been found, including chenodeoxycholic acid, flavanone and 2′-hydroxyflavanone (S. Diederich et al. Eur. J. Endocrinol. 2000, 142, 200 and R. A. S. Schweizer et al. Mol. Cell. Endocrinol. 2003, 212, 41).
A need exists in the art for 11β-HSD1 inhibitors that have efficacy for the treatment of diseases such as type II diabetes mellitus and metabolic syndrome. Further, a need exists in the art for 11β-HSD1 inhibitors having IC50 values less than about 1 μM.